The present invention is related in general to the field of detecting materials and specifically to an ion detection device with a gaseous sample inlet and ionizer that provide for increased sensitivity in the detection of traces of materials in the vapor phase. The device of the present invention has achieved superior sensitivity to vapor phase analytes without either the use of condensation or concentration of the vapors as liquids or solids and without the collection of analyte material as droplets or particles. Used in conjunction with condensation or concentration of analyte materials, even greater sensitivity could be achieved.
The rapid identification of explosives, explosive residues, chemical agents, airborne toxins, and other volatile organic compounds has undergone a revolution in recent years by the progress made in the field of ion mobility instruments. Despite the transformation that has occurred in ion mobility spectrometry, the full potential of the technique has not yet been realized, particularly in the analysis of gases. This is partially due to the low numbers of ions generated in the small ionizers employed in conventional ion mobility spectrometers. As will be appreciated by one of ordinary skill in the art, existing devices are limited in detecting traces of materials by the low number of ions generated by the materials in a ionization chamber because the existing devices require a certain number of ions (above a threshold) to be present in order to detect the materials from which the ions originated.
FIG. 1 shows a typical ion mobility spectrometer (IMS) that includes an ionization region 1 and a reaction chamber 10 in which a gas 7 enters and is ionized, an ion drift chamber 15 coupled in series with the reaction chamber 10 through an ion injection shutter 12, and a collector plate 16 disposed inside the drift chamber 15, opposite the injection shutter 12. In operation, a carrier gas transports gases or vapor from a material to be analyzed into an ionization region 1 containing an ionization source 2. A repeller electrode 4 (also called a pusher electrode), which may be in the form of a plate or a screen, is provided in the ionization region to direct ions toward the drift chamber. Most of the resulting ions (primary ions) are from the carrier gas molecules (“reactive or reactant ions”), which move to the reaction chamber (10) where multiple collisions occur between ionized species and the analyte molecules. These collisions transfer ion charges to the analyte molecules forming secondary ions. It is also known in the art to employ a reagent gas which is introduced into the reaction region so that secondary ions are predominantly formed from the reagent interacting with primary ions. In this case, sample is also introduced into the reaction region and charge is transferred from the primary and secondary ions to the analyte (tertiary ions.) All ions move, predominantly, by “electrophoresis” in the electric field inside the spectrometer. The electric field, formed by conventional techniques, moves ions from the reaction chamber 10 to the drift chamber 15 and ultimately to reach the collector plate 16. Typically, a drift gas is introduced into the drift chamber and exits through gas exit 9. The combined portions of the apparatus, outside the ionizer, where ions move by electrophoresis are called, generically, the “drift tube.” Hill, H. H. et al. (1990) “Ion Mobility Spectrometry,” Anal. Chem. 62(23):1201A-1209A and Eiceman, G. A., Karpas, Z. (2005) Ion Mobility Spectrometry, (CRC Press) provide reviews of ion mobility spectrometry, including instrumentation.
U.S. Pat. No. 4,777,363 reports an atmospheric ion mobility spectrometer for detection of trace substances in ambient air. In this spectrometer the air acts as the sample, carrier and drift gases. In a “uniflow design,” ambient air is introduced through an inlet at the collector end of the drift tube and a gas exit with a pump is provided at the opposite end of the instrument beyond the ion source. A repeller plate is provided at the end of the instrument having the drift gas exit. Ions formed at the ion source and secondary ions formed on reaction move into the drift chamber as ion pulses formed at an ion shutter. Ions in the drift chamber move in a direction opposite the direction of flow of the air drift gas. Ions are detected at the collector plate. The reference describes the use of a Ni-63 (radioactive source) ion source. The reference describes a “typical Ni-63 source” as a cylinder of Ni (1 to 2 cm in diameter and 1-2 cm long with surface area of 3-5 cm2) with Ni-63 plated on the inner surface. It is stated that a “major limitation in the linear response range has been attributed to a limited availability of ions from the ion source.” The reference describes a higher activity ion source as “a nickel slug approximately 3 to 5 cm in diameter with a plurality of holes for Ni-63 plating.” This ion source is said to provide “a much higher surface area and activity rate without requiring an increase in the size of the ion source.” A photoionization source is also described. The instrument is said to be pneumatically sealed with a pump on the repeller end.
U.S. Pat. No. 5,218,203 reports a high pressure interface device for introducing sample ions to a drift tube of a ion mobility measurement means. The patent describes an “isolated” ionization source which is illustrated in FIG. 1 of the patent to be at the ion source gas (B1) inlet. It is stated that “it is important that the ionization of the ion source gas occurs in an isolated region where no sample gas is present.” The sample gas ions formed are described as introduced into a second reaction region where they react with a sample gas (B2) to form sample ions. The device configuration used is described as minimizing or eliminating introduction of unwanted components into the drift tube. The patent discusses the presence of an electric field E to direct the ion source gas ions through the flow path of the sample gas.